Q:
A: The period you are referring to are the three weeks of mourning between the 17th of Tammuz and the 9th of Av. It is a time when we are in mourning for the destruction of the Temples (both were destroyed on the 9th of Av). It starts ion the 17th of Tammuz since it is on this date that the Romans breached the walls of Jerusalem and the siege of the Temple proper started. Also, various other calamaties happened on it- amongst them: the 17th of Tammuz is the date on which Moshe broke the first set of Tablets he got from Hashem on Mt Sinai. The 17th of Tammuz is considered a minor fast day in that we fast from sunrise to sunset. The 9th of Av is considered a major fast day on which we fast from sunset to sunset. Throughout the three weeks we do not listen to music, hold weddings or other celebrations and avoid buying new things or getting involved in legal matters (since it is a time of bad luck, rather avoid such situations). from the 1st to the 9th of AV, we do not eat meat either.